The present invention relates to an implantable prosthesis for implantation in a bone such as a femur.
Applicant""s prior U.S. Pat. No. 6,273,915 (the ""915 patent), which is incorporated herein by reference, is directed to a femoral head-neck prosthesis and method of implantation. The prosthesis comprises a neck mounted on a collar which rests against the femoral neck. A stem is mounted on the underside of the collar to extend generally downward through the femur. In the method of implantation, a seat is formed in the femoral neck to engage the underside of the collar. The shape of the seat is complementary to that of the underside of the collar. If the underside of the collar and the seat are congruent, the entire area of the seat should engage the underside of the collar so that there is 100 percent bone contact at the interface for transmitting substantially 100 percent of the loading. However, greater force may be transmitted to the bone at the intersection of the stem and collar due to line contact between the bone and the stem-collar intersection. According to Wolff""s law, changes in stress or force distribution eventually cause alterations in the internal structure of the bone. Those portions of the bone subject to a lesser stress than before are likely to deteriorate and those subject to greater stress than before are likely to thicken. Accordingly, if there is greater stress at the stem-collar intersection, bone there will thicken and will also accumulate so that stress at the intersection steadily increases over time, while stress transmission through the collar at locations spaced from the intersection steadily decreases and bone at such locations deteriorates or xe2x80x9cresorbsxe2x80x9d. The bone may resorb to such an extent that replacement of the implant will be required.
Ideally, a femoral prosthesis should allow for a broad range of motion after implantation. Conventional prostheses may limit the range of hip motion because the neck of the prosthesis impinges or contacts the acetabulum when the femur is pivoted to some positions. Thus, it would be desirable if the neck could be formed to allow a broader range of motion.
In a non-cemented femoral prosthesis, the prosthesis should be fixed in the bone to inhibit movement about axes perpendicular to the longitudinal axis of the prosthesis. As disclosed in the ""915 patent, the stem of the prosthesis may include splines to suitably fix the prosthesis. It would be desirable to include additional structure for fixing the prosthesis in the bone, e.g., to further inhibit lateral and medial movement about axes perpendicular to the longitudinal axes.
Among the several objects and features of the present invention may be noted the provision of a prosthesis which promotes force transmitting engagement of the prosthesis with the bone; the provision of such a prosthesis which inhibits line contact between the prosthesis and bone; the provision of such a prosthesis which inhibits bone resorption; the provision of such a prosthesis which allows a significant range of motion of the joint after implantation; and the provision of such a prosthesis which inhibits prosthesis movement about axes perpendicular to the longitudinal axis after implantation.
Briefly, apparatus of this invention is a prosthesis for implantation in a bone comprising a collar having a first surface and a second surface generally on an opposite side of the collar for engaging the bone and transferring forces to the bone. A neck extends outwardly from the first surface and is adapted to receive a ball thereon. A stem extends outwardly from the second surface of the collar for reception in the bone. The second surface is shaped to promote force transmitting engagement of the second surface with the bone over at least a substantial portion of the second surface, to limit engagement of the second surface with the bone at an area of stress concentration in the second surface, and to inhibit line contact between the second surface and bone.
In another aspect of the invention, a femoral prosthesis comprises a collar having a first surface and a second surface generally on an opposite side of the collar for engaging the femur and transferring forces to the femur. A neck extends outwardly from the first surface and is adapted to receive a ball thereon. A stem extends outwardly from the second surface of the collar for reception in the femur. The second surface includes a recess adjacent to the stem to promote force transmitting engagement of the second surface with the femur over at least a substantial portion of the second surface and to inhibit contact with the femur generally at a location where the stem intersects the collar to thereby inhibit bone resorption.
In yet another aspect of the invention, the femoral prosthesis comprises a neck including a recessed portion to inhibit contact with the acetabulum upon implantation in the femur.
In still another aspect of the invention, the femoral prosthesis comprises a collar including a laterally outwardly facing surface extending generally perpendicular to the first surface and the second surface. The surface has a partial cylindrical shape generally complementary to a resected portion of the lateral neck to thereby inhibit movement about axes perpendicular to the longitudinal axis upon implantation in the femur.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.